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The means through which data is transformed from one place to another is called transmission or communication media. There are two categories of transmission media used in computer communications.

Bounded/Guided Media

Uubounded/Unguided Media

Bounded Media

Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up o a external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are

Coaxial Cable

Twisted Pairs Cable

Fiber Optics Cable

Coaxial Cable

Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.

Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.

Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.

Outside the copper mesh is the final protective cover. (as shown in Fig)

The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.

Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core.

Here the most common coaxial standards.

50-Ohm RG-7 or RG-11 : used with thick Ethernet.

50-Ohm RG-58 : used with thin Ethernet

75-Ohm RG-59 : used with cable television

93-Ohm RG-62 : used with ARCNET.

Characteristics of Coaxial Cable

Low cost

Easy to install

Up to 10Mbps capacity

Medium immunity form EMI

Medium of attenuation

Advantages

Inexpensive

Easy to wire

Easy to expand

Moderate level of EMI immunity

Disadvantages

Single cable failure can take down an entire network

Twisted Pair Cable

The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive and support many different types of network. It also supports the speed of 100 mps. Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire gauge standard. There are two types of twisted pairs cabling

Unshielded twisted pair (UTP)

Shielded twisted pair (STP)

Unshielded twisted pair (UTP)

UTP is more common. It can be either voice grade or data grade depending on the condition. UTP cable normally has an impedance of 100 ohm. UTP cost less than STP and easily available due to its many use. There are five levels of data cabling

Category 1

These are used in telephone lines and low speed data cable.

Category 2

These cables can support up to 4 mps implementation.

Category 3

These cable supports up to 16 mps and are mostly used in 10 mps.

Category 4

These are used for large distance and high speed. It can support 20mps.

Category 5

This is the highest rating for UTP cable and can support up to 100mps.

It is similar to UTP but has a mesh shielding that’s protects it from EMI which allows for higher transmission rate.

IBM has defined category for STP cable.

Type 1

STP features two pairs of 22-AWG

Type 2

This type include type 1 with 4 telephone pairs

Type 3

This type feature two pairs of standard shielded 26-AWG

Type 4

This type of STP consist of 1 pair of standard shielded 26-AWG

Type 5

This type consist of shielded 26-AWG wire

Characteristics of STP

Medium cost

Easy to install

Higher capacity than UTP

Higher attenuation, but same as UTP

Medium immunity from EMI

100 meter limit

Advantages of STP:

Shielded

Faster than UTP and coaxial

Disadvantages of STP:

More expensive than UTP and coaxial

More difficult installation

High attenuation rate

Fiber Optics

Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse of light through this cable to other device. These pulses translated into “1’s” and “0’s” at the other end.

In the center of fiber cable is a glass stand or core. The light from the laser moves through this glass to the other device around the internal core is a reflective material known as CLADDING. No light escapes the glass core because of this reflective cladding.

Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second)

Characteristics Of Fiber Optic Cable:

Expensive

Very hard to install

Capable of extremely high speed

Extremely low attenuation

No EMI interference

Advantages Of Fiber Optic Cable:

Fast

Low attenuation

No EMI interference

Disadvantages Fiber Optics:

Very costly

Hard to install

Unbounded / Unguided media

Unbounded / Unguided media or wireless media doesn't use any physical connectors between the two devices communicating. Usually the transmission is send through the atmosphere but sometime it can be just across the rule. Wireless media is used when a physical obstruction or distance blocks are used with normal cable media. The three types of wireless media are:

Radio Waves

Micro Waves

Infrared Waves

Radio waves

It has frequency between 10 K Hz to 1 G Hz. Radio waves has the following types.

Short Waves

VHF (Very High Frequency)

UHF (Ultra High Frequency)

Short Waves

There are different types of antennas used for radio waves. Radio waves transmission can be divided into following categories.

Low Power, Single Frequency.

High Power, Single Frequency

Micro Waves

Micro waves travels at high frequency than radio waves and provide through put as a wireless network media. Micro wave transmission requires the sender to be inside of the receiver.

Following are the types of Micro waves.

Terrestrial Micro waves

Satellite Micro waves

Terrestrial Micro waves

Terrestrial Micro waves are used are used to transmit wireless signals across a few miles. Terrestrial system requires that direct parabolic antennas can be pointed to each other. These systems operate in a low Giga Hertz range.

Characteristics of Terrestrial Micro waves

Moderate to high cost.

Moderately difficult installation

1 M bps to 10 M bps capacity

Variable attenuation

Low immunity to EMI

Satellite Micro Waves

The main problem with aero wave communication is the curvature of the earth, mountains & other structure often block the line of side. Due to this reason, many repeats are required for long distance which increases the cost of data transmission between the two points. This problem is recommended by using satellites.

Satellite micro wave transmission is used to transmit signals through out the world. These system use satellites in orbit about 50,000 Km above the earth. Satellite dishes are used to send the signals to the satellite where it is again send back down to the receiver satellite. These transmissions also use directional parabolic antenna’ with in line of side.

In satellite communication micro wave signals at 6 GHz is transmitted from a transmitter on the earth through the satellite position in space. By the time signal reaches the satellites becomes weaker due to 50,000 Km distance. The satellite amplifies week signals and transmits it back to the earth at the frequency less than 6 GHz.

Characteristics Satellite Micro Waves

High cost

Extremely difficult and hare installation.

Variable attenuation.

Low immunity to EMI

High security needed because a signal send to satellite is broadcasts through all receivers with in satellite.

Infrared

Infrared frequencies are just below visible light. These high frequencies allow high sped data transmission. This technology is similar to the use of a remote control for a TV. Infrared transmission can be affected by objects obstructing sender or receiver. These transmissions fall into two categories.

Point to point

Broadcast

Point to Point

Point to point infrared transmission signal directly between two systems. Many lap top system use point to pint transmission. These systems require direct alignment between many devices.

Characteristics of Point to point

Wide range of cost

Moderately easy installation.

100 k bps to 16 Mb of capacity.

Variable attenuation.

High immunity to EMI

Broad Cast

These infrared transmission use sprayed signal, one broad cast in all directions instead of direct beam. This help to reduce the problems of proper alignment and abstraction. It also allows multiple receiver of signal